/Engine/Source/Editor/UnrealEd/Public/FileHelpers.h
Opens a non-modal dialog to allow the user to choose a level
static void OpenLevelPickingDialog ( const [FOnLevelsChosen](API\Editor\UnrealEd\FEditorFileUtils\FOnLevelsChosen) & OnLevelsChosen, const [FOnLevelPickingCancelled](API\Editor\UnrealEd\FEditorFileUtils\FOnLevelPickingCancelled) & OnLevelPickingCancelled, bool bAllowMultipleSelection )